Gage-control system for strip mill



March 23, 1965 J. B. CAMP 3,174,317

GAGE-CONTROL SYSTEM FOR STRIP MILL Filed Sept. 29. 1961 2 Sheets-Sheet 1 7TB 1 INVENTOR JAMES B. CAMP imwzwv A Horney March 23, 1965 J. B. CAMP 3,174,317

GAGE-CONTROL. SYSTEM FOR STRIP MILL Filed Sept. 29, 1961 I 2 Sheets-Sheet 2 INVENTOR JAMES CAMP x a/awzv Attorney Unitcd States Patent 3,174,317 GAGE-CGNTROL SYSTEM FOR STRH MILL James B. Camp, Fair'iield, Ala, assignor to United States Steel Corporation, a corporation of New Jersey Filed Sept. 29, 1961, Ser. No. 141,830 7 Clm ms. (Cl. 72-12) This invention relates generally to continuous striprolling mills and, in particular, to a system for controlling the gage of the strip produced thereby in order that the product will be more uniform in thickness and, at the same time, will conform to the desired sectional shape or profile.

fanual control of rolling-mill screws and of the speeds of the stand motors to maintain uniform gage has been superseded by an automatic system in which a gager between the first and second stands of a multi-stand mill controls the screws of the first stand. This is only a partial solution of the problem because of the time factor involved. Automatic control of the speed of the last stand (and therefore the tension between the next-to-last and last stands) has also been tried, governed by a gager measuring the thickness of the finished strip. This is satisfactory where only a slight reduction is efiected in the last stand and strip shape or cross-sectional profile is not critical. These conditions, however, exist for Only a small raotion of strip products. It is accordingly the object of my invention to provide a novel and more efiicient gagecontrol system.

According to my invention, the necessary gage corrections are elfected primarily by varying the speed of the first stand to vary the tension on the strip between the first and secondstands. To actuate this control, I employ a gager measuring the thickness of the strip as it emerges from the second stand. secondarily, I provide for controlling the screws of the second stand to keep the tension control within its practical operating range. I also use the finished gage of the strip to modify the primary control of interstand tension, by applying thereto a Vernier control and feedback. Finally, I effect an anticipatory infiuence on the tension between the first two stands in accordance with the strip thickness at that point.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawings illustrating the present preferred embodiment. In the drawings:

FIGURES l and 2 together constitute a schematic diagram showing the system as a whole and its relation to the stands of a continuous strip mill; and

FEGURE 3 is a perspective view of a memory device incorporated in the system.

The drawings show circuits largely in single line. Mechanical connections or drives between electrical units are shown in dotted lines.

Referring now in detail to the drawings, and particularly to FIGURES l and 2, the system of my invention is there shown as applied to a four-stand continuous strip mill composed of stands 1, 2, 3 and 4, an uncoiler 5 feeding strip into'stand l and a coiler 6 winding up the strip issuing from stand 4. Each mill stand is driven by an individual motor. The motors are shown at 7, 8, 9 and 10 and are directly connected to and supplied individually by generators 11, 12, 13 and 14. The field winding of generator 11 is shown at 11a. Motor 7 has its field winding 15 connected to an exciter 16. The field winding of exciter 16 is shown at 17. Generators 13 and 14 have field windings shown at 18 and 19, respectively. Stands 1 through 4 have motorized screwdowns. The motor for driving the screwdowns of stand 2 is indicated at 20.

The primary control system On the exit sides of stands 1, 2 and 4 are located similar gagers 21, 22 and 23. These are Measuray X-ray thickness gagers of known construction manufactured by Southeastern Electronics Laboratories, Urnatilla, Florida. For the moment, only gager 22 need be considered. Gager 22 governs the primary or basic gage-control portion of the system of my invention. It functions to set up an output D.C. voltage varying in magnitude as a function of the percentage deviation of strip thickness from a manually pre-set value and in polarity as a function of the direction of deviation, i.e., heavy gage or light gage. Gager 22 includes a zero-adjustment potentiometer 22a, the purpose of which will appear later. i

The voltage output of gager 22 is applied to a selector circuit or logic 25. The circuit comprises a pair of parallel paths including manually adjustable rheostats 26 and 27 in series with unidirectional conducting devices 28 and 29 (diode or other rectifier) and normally closed relay contacts 39 and 31. Contacts 39 and 31 are actuated by relays 32 and 33, under the control of a tension indicator 34, and remain closed so long as the strip tension between stands 1 and 2 does not vary beyond predetermined limits. This indicator is a conventional device manufactured by Westinghouse Electric Corporation and is described and illustrated in Instruction Book 350060 published by said manufacturer. When the tension exceeds a'predetermined maximum, at contact 34a is closed, energizing relay 33 and thereby opening contact 31. When the tension falls below a predetermined minimum, a contact 34b is closed energizing relay 32 and thereby opening contact 30. Relays 32 and 33 are connected in series with switches 34b and 34a, respectively, across an AC. source 53. As shown, the devices 28 and 29 are reversed relative to each other so one conducts current of one polarity and the other conducts current of reverse polarity. That is to say, the polarity of the voltage from gager 22 determines which of the parallel paths of circuit 25 will conduct. Rheostats 26 and 27 are set at positions corresponding to the minimum and maximum thickness of strip desired. It the strip tension between stands 1 and 2 is either too high or too low to begin with, contact 31 or contact 3% will open, to prevent any adverse change in the tension. Logic 25 thus relates the need for gage correction to the existing condition of strip tension.

The D.C. voltage output of gager 22, under the control of logic 25, is converted by known means to an AC. voltage, varying in phase depending on the polarity of the signal received, then amplified and applied to the motor of potentiometer 35. This is a Bristol Company Series 565 Round-chart Dynamaster A.C. bridge modified to operate as a servo-integrator, i.e., the output of the potentiometer is substantially the integral of the input thereto. This output is applied to an electro-mechanical power amplifier 36 composed of a rotating device (General Electric Company Amplidyne) feeding a second similar unit (Westinghouse Electric Corporat on Rototrol). The Rototrol amplifier is an integral part of the mill-stand voltage regulator system and the Amplidyne amplifier is a means of inserting a control signal into the mill regulator. The output of amplifier 36 energizes the field winding 11a of generator 11.

The position of the moving contact of potentiometer 35 thus varies the output of amplifier 36 which energizes the field winding 11a. of generator '11. It will be evident that connection of the circuits described may be made so that a heavy-thickness signal from gager 22 will reduce the excitation of generator 11 and, therefore, the speed of motor 7 while a light-thickness signal will have the reverse effect. The circuits described thus cause adjustment of the tensionbetween stands 1 and 2 to compensate for gage deviations measured by gager 22. The gain adjustment of amplifier 36 is operated by conventional means (not shown) which varies the excitation of the field windings of all generators 11, 12, 13 and 14 simultaneously.

.If potentiometer 35 operates through a predetermined angle in one direction or the other, indicating correction for excessive deviation from the desired gage, one of the limit switches 371-1 and 37L in a logic 37 is closed. This initiates, through means now to be described, a direct effect on the field winding of exciter 16, to further control the speed of motor 7.

Logic 37 has a D.C. power supply 38 and includes back contacts 30a and 31a operated by relays 32 and 33. Assuming normal tension on the strip between stands 1 and 2, these contacts are closed and closure of either switch 37L or 37H charges or discharges a condenser 37C through a resistor 37R. Thus, a variable voltage is applied to an amplifier 39, the output of which energizes the field winding 17 of exciter 16. Amplifier 39 comprises a rotating electro-mechanical amplifying device (General Electric Company Amplidyne) and conventional associated electronic equipment. An increase in the output of logic 37 will increase the speed of motor 7 while a decrease in the output of logic 37 will decrease the speed of motor 7. This follows from the fact that switch 37L is closed when correction for light gage is needed and applies voltage to condenser 37C, while switch 37H is closed when correction for heavy gage is needed and permits discharge of condenser 370.

Thus logic 37 functions to determine when the field winding of motor 7 should be directly adjusted. For example, if potentiometer 35 is operating to correct for light gage, the output of logic 37 is increased if limit switch 37L is closed. This increases the speed of motor 7 by reducing the net excitation of its field winding 15. If the strip tension is below the minimum, however, relay 32 is energized to open contact 30a and the closing of switch 37L will have no eifect.

In addition to minor corrections to strip gage by control of the speed of motor 7 as already explained, I provide for changing the setting of the mill screws of stand 2 when necessary to make a more drastic correction. For this purpose, I employ a mill-screw bumping logic 40 controlled by a polarized relay 41 responsive to the direction and extent of strip-gage deviation from normal as evidenced by the output of gager 22. If the gage is more than 1% light, relay 41 closes its contact 41a. If the gage is more than 1% heavy, it closes its contact 4111. Contacts 30b and 31b operated by relays 32 and 33 are in series, respectively, with contacts 41a and 41b, so both relay 41 and indicator 34 jointly control the screw-motor operation.

When contacts 3% and 41a are both closed, a relay 42 is energized which causes the screw motor to back oil the millscrews. When contacts 31b and 41b are closed, a relay 43 is energized which causes motor 20 to turn the mill screws down. Relays 42 and 43 are incorporated in a conventional reversing-control panel 20a for motor 20. They are energized only momentarily, however, so as to bump the screw motor slightly, because they are in series with the contact 44 of an on-off timer 45, which is closed only during the on time. The timer is a standard asymmetrical multi-vibrator type, a conventional device, and is energized for one cycle from a DC. source 46 th ough contacts 300 and 310, only when either one is closed by energization of either one of relays 32 and 33.

A ratchet lock-out relay 47 is in parallel with relay 42. After a predetermined number of operations, it opens its back contact 47x, in series with relay 42 and prevents further operation thereof until reset. A similar relay 48 operates its back contact 48x in series with relay 43. The operations sequence is as follows: With the gage 1% or more too low, contact 41a will be closed by relay 41. When the strip tension reaches its lower limit, contact 34b closes, energizing relay 32. The resulting closure of con- 4 tact 30c initiates one cycle of timer 45 and the closing of its contact 44 completes the circuit for relays 42 and 47. A similar operation of relays 43 and 48 results from the occurrence of heavy gage and opposite reaction of relay 41. If one bump of the screws is not sufiicient correction, repeated bumps will occur until relays 47 or 48 open their back contacts. This precludes further bumping until the relays 47 and 48 are reset.

By way of explanation of the system described so far, it should first be noted that the two primary ways of effecting a change in the strip thickness are by changing (1) roll pressure or (2) interstand tension. Neither is sufficient alone, therefore the primary or basic gage control combines both. The tension variation is used primarily to control the strip thickness and roll pressure is varied to keep the strip within the allowable ranges of the tension control. It should also be observed that an increase in the back tension on stand 2 will result in a decrease in the strip thickness leaving the stand. The reverse occurs if the tension is decreased. The forward tension on stand 1 is, of course, equal to the back tension on stand 2. The change in forward tension on stand 1, however, has little or no effect upon the strip thickness leaving that stand.

Vernier Control and Feedback Having described the primary or basic gage-control system forming part of my invention, I shall now explain means affording a further refinement in gage control which I designate the vernier control and feedback. This portion of the system is responsive to gager 23 located on the exit side of stand 4. Gager 23 supplies a control signal like that developed by gager 22 to a potentiometer 50 which is a Bristol Company Series 565 instrument modified in the same manner as potentiometer 35. Potentiometer 50 controls duplicate amplifiers 51 and 52 both similar to amplifier 36. Amplifier 51 supplies the field Winding 18 of generator 13. In short, amplifier 51 efiects control of the speed of stand 3 in substantially the manner that amplifier 36 controls the speed of stand 1. Amplifier 52 similarly controls the speed of stand 4. Gager 23, however, also effects a readjustment of the control exerted by gager 22.

To this end, potentiometer 50 has limit switches 50L and 50H which are closed when it has turned through 25% of its angular travel in one direction or another to correct for light or heavy gage as measured on the exit side of stand 4. These switches control a reversible servo-motor 55 driving rheostat 22a which is the manual setting for gager 22. Operation of motor 55 is limited to short periods by a relay 56, the front contact 56a of which is in series with limit switches 50L and 50H. Relay 56 is energized during the on time of an on-otf timer 57. Timer 57, similar to timer 45, determines the rate and duration of the corrective adjustments which are fed back to gager 22. This feedback is accomplished by changing the manualsetting or zero-adjustment potentiometer 22a of gager 22. The on-ofi timer 57 is the free-running type, the on time of which is constant and the OE time which is varied inversely with the speed of the mill, by a tachometer generator 58 driven by motor 9.

During operation, if the potentiometer 50 is caused to rotate 25% of its etfective travel from the zero position to correct a light-gage condition, limit switch 50L will be closed. When the on-01f timer 57 is in the on condition, relay 56 is energized and the servo-motor 55 is caused to rotate. This varies the position of rheostat 22a or the manual set point of gager 22 so that the primary gage control will now regulate around a slightly heavier gage. A reverse operation of motor 55 will occur, by

closure of switch 50H, to correct for heavy gage. Essentially, the vernier control and feedback system just explained detects an error in the thickness of strip leaving stand 4 and initiates a correcting signal. This signal is amplified by suitable means and causes a speed change which varies the back tension on strip entering stands 3 and 4, through speed control of motors 9 and 10 and ultimately brings the strip thickness back to the point where the error signal becomes zero. Because of the lag between correction and subsequent gaging of the corrected portion of the strip, the response of the system must be relatively slow, and since the vernier control and feedback system acts only as a fine control, any errors in gage of suthcient magnitude will cause a corrective signal to be sent back to gager 22.

Anticipatory circuit Finally, my invention includes means for introducing an anticipatory eflect on the primary gage-control system in accordance with departures from desired gage after the strip leaves stand 1. For this purpose, gager 21 delivers to a power amplifier 60 of known type a signal proportional to the deviation of the strip thickness from the desired value and the amplified signal is recorded on a magnetic memory disc 61 by a recording coil 62. Disc 61 is driven at a speed proportional to that of stand 1, say 50%. The position of coil 62 as shown in FIGURE 3, is adjustable to provide a variable time delay for the travel of an ofi-gage portion of the strip from stand 1 to stand 2 and permit the application of a corrective adjustment to the speed of stand 1, during such travel. The adjustment is initiated by a fixed pick-up or receiving coil 63 cooperating with disc 61.

The drive for disc 61 is indicated in FIGURE 3 by gears 64 and shaft 65. Coil 62 is mounted on a crank arm 66 extending outwardly from a sleeve 67 coaxial with shaft 65. A reversible servo-motor 68 drives sleeve 67 through a worm-and-worm-wheel 69. An erasing magnet 70 adjacent disc 61 wipes out the record made by coil 62 after it has been picked up by coil 63. The operation of motor 68, as shown in FIGURE 1, is under the control of a polarized relay 71. Normally the relay is in mid-position because of the balance between a voltage from a supply source 72 and that of a tachometer 73 driven by motor 7, applied differentially. In case of a difference between said voltages, relay 71 closes one or the other of its front contacts 71a and 71b, energizing motor 68 to drive a potentiometer 74 in such direction as to restore the balance. The motor also drives sleeve 67.

Thus the position of recording coil 62 is varied proportionally to any rotation of the servo-motor 68. During operation, any change in the speed of the motor driving stand 1 will effect a change in the output of tachometer 73, thus causing a potential diiference across polarized relay 71. This in turn closes contact 71a or 71b, de ending upon the polarity of the potential difierence. The closing of one of these contacts causes the servo-motor to run in the forward or reverse direction to balance out the potential difference across relay 71 by varying the position of the potentiometer 74. The movement of the servo-motor causes a proportional peripheral movement of recording coil 62. Recording coil 62 thus assumes a different position for each speed of the mill and maintains a constant time delay between the instant the error signal is received by coil 63 and the time the elf-gage portion of the strip enters stand 2. The signal picked up from disc 61 by coil 63 is applied to an amplifier 75 which demodulates the signal and applies it to amplifier 36.

Gager 22 and the instrumentalities controlled thereby correct for an erroneous trend in the thickness of the strip leaving stand 2. Short-term deviations in gage may pass beyond stand 2 before the corrective effect can be brought about, because of the inevitable time lag caused by inductance, inertia, required transport time, etc., but such errors are noted by gager 21 and the anticipatory circuit controlled thereby sets up the corrective measures in advance so they will be effective by the time the offgage portion of the strip reaches stand 2.

The anticipatory circuit operates as follows. When the strip leaving stand 1 is elf-gage, or changing to elf-gage, the error is detected by gager 21 and an error signal is means 61. By these means, the signal is differentiated, amplified and delayed for an interval of time which is inversely proportional to the speed of the mill; Following the delay and further amplification by amplifier 75, the signal is fed into amplifier 36 where it is etfective in exactly the same manner as the error signals originating in gager 22. The amplification of the error signal from gager 21 is controlled so that, if the same error existed at gager 22 and gager 21, the resulting signals fed to power amplifier 36 would be identical except for the trend of the change in strip thickness. The interval of delay in making the necessary adjustment caused by gager 21 is determined by the speed of the motor of stand 1 and is of sufficient duration to allow the necessary correction to be made before the off-gage portion of the strip reaches stand 2. Thus, the anticipatory circuit gives the'gagecontrol system the ability to prepare in advance to correct off-gage portions by the time they arrive at stand 2. Therefore, such portions which could otherwise pass or be accentuated may, with this added refinement, be eliminated or substantially reduced.

Although I have disclosed herein the preferred embodiment of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

l. A gage-control system for a multi-stand strip mill including a motor for driving each stand and a generator for supplying each motor, comprising a gager located to measure the thickness of the strip as it issues from the second stand, means responsive to said gager effective to vary the excitation of the first-stand generator, means actuated on operation of said excitation-varying means to a predetermined position, to vary the excitation of said first-stand motor.

2. The combination defined in claim 1, characterized by a tension indicator engaging the strip between the first and second stands and means actuated by said indicator to prevent operation of said motor-excitation varying means when the tension on the strip between the first and second stands is above a predetermined maximum or below a predetermined minimum.

3. A gage-control system for a multi-stand strip mill including a motor for driving each stand and a generator for supplying each motor, comprising a gager located to measure the thickness of the strip as it issues from the second stand, means responsive to said gager effective to vary the excitation of the first-stand generator, a tensionresponsive means engaging the strip between the first and second stands and means actuated by said tension-responsive means to prevent operation of said excitationvarying means when the tension of the strip is above a predetermined maximum or below a predetermined minimum, said gager including an adjustable rheostat, a second gager located to measure the thickness of the strip issuing from a mill stand subsequent to the second, and a motor controlled by said second gager eifective to drive said rheostat.

4. The combination defined in claim 3, characterized by means limiting operation of said rheostat-driving motor to a variable portion of successive time intervals.

5. The combination defined in claim 4, characterized by means varying the extent of said variable portion according to the speed of the next-to-last stand.

6. The combination defined in claim 3, characterized by means controlled by said second gager to vary the excitation of the generator of the last stand of said mill and the one next-to-last.

7. A gage-control system for a multi-stand strip mill including a motor for driving each stand and a generator for supplying each motor, comprising a gager located to measure the thickness of the strip as it issues from the second stand, means responsive to said gager effective to vary the excitation of the first-stand generator, a tension responsive means engaging the strip between the first and References Cited in the file of this patent second stands and means actuated bysaid tension-respon- UNITED STATES PATENTS 'sive means to prevent operation of said excitation-varying means'when the tension on the strip is above a predeter- 2,883,895 v9ssberg P 1959 mined maximum or below a predetermined minimum, a 5 2,988,680 Dlrth June 13, 1961 second gager located to measure the thickness of the strip 31030336 Gochenour P 1962 issuing from the first stand and means controlled by said second gager for varying the excitation of the first-stand OTHER REFERENCES generator according to variations in strip thickness after Control Engineering, September 1956, pages 116 and a time interval inversely proportional to the speed of the 10 17 (C py in Scientific Library).

first stand. 

1. A GAGE-CONTRL SYSTEM FOR A MULTI-STAND STRIP MILL INCLUDING A MOTOR FOR DRIVING EACH STAND AND A GENERATOR FOR SUPPLYING EACH MOTOR, COMPRISING A GAGER LOCATED TO MEASURE THE THICKNESS OF THE STRIP AS IT ISSUES FROM THE SECOND STAND, MEANS RESPONSIVE TO SAID GAGER EFFECTIVE TO VARY THE EXCITATION OF THE FIRST-STND GENERATOR, MEANS ACTUATED ON OPERATION OF SAID EXCITATION-VARYING MEANS TO A PREDETERMINED POSITION, TO VARY THE EXCITATION OF SAID FIRST-STAND MOTOR.. 